I use vmware server a lot and unfortunately this happens way too frequently. I'm not sure if the bug lies in Firefox or VMware but there should be a username and password entry box in the middle of that screenshot.
So far I have not found a cure for it.
I do have workaround however, but it's not a pretty one.
Basically, what you have to do is hit
ctrl+shift+rquickly and repeatedly until the dialog appears. Stop every half a dozen or so presses to let it catch up and hopefully the dialog will appear. If not, rinse, lather, repeat until it does.
Hardly an elegant solution I know, but at least you will be able to log into your console.
P.S. Performing the same procedure using F5 does not produce the same result (well, not for me anyway)